Unexpected watercolor effect from underglaze and clear overglaze; it's pretty but what happened?

[LilBlueFrog]

Hi, I've only been doing pottery for a few years, and am trying to learn how to not ruin things with glaze! 

I made this mug out of B mix that I THINK was cone 5. I applied mayco stroke and coat to the leather hard piece, carved through it, and bisqued it.  It looked as I expected, with the underglaze where it was intended to be. There was even a fleck of it that had dripped where I didn't want it, and I could not get it off with sanding. So I felt I could assume the underglaze was well fused to the clay body.

The handle was originally without underglaze but I decided I wanted it purple as well, and figured then I'd get to see the difference between stroke and coat on greenware vs on bisque. I then applied 1 dip coat of Coyote Clear MBG007-P and fired to cone 5/6 (don't know which exactly, my clay supplier fires things for me).

I did not expect the glaze and underglaze to interact,  I did not expect the glaze to run, and I did not expect the glaze to craze, but all of these things happened (the running I am basing on the fact that it stuck a little to the kiln shelf). The difference between underglaze on bisque vs greenware was unremarkable, both gave a mottled watercolor effect. 

It's pretty, but what happened? Could this be a result of this clay actually being cone 10? Perhaps it crazed because I applied it too thickly?

I am more of an organic chemistry person, but inorganic is certainly prettier, and I am excited to learn things from you all!

Edited September 13, 2022 by LilBlueFrog
Forgot apostrophes in it's

[Callie Beller Diesel]

Inorganic chemistry is also pretty fun, I agree! 

Short answers:

1)The floating thing is what happens when your underglaze* gets dissolved by your glaze: bits and pieces of the purple are floating in the clear.  On your mug you’re  mixing 2 distinct glass relatives when they’re hot, but they’re not spending enough time at top temperature to become homogeneous. If you give them more time to melt together in, say, a second firing, they start mixing together more. This accounts for the more floating look of the combo on the body vs the less incorporated look on the handle. 

2)The crazing thing is because the overglaze doesn’t fit your clay body: it’s unlikely anything you did or didn’t do affected this. They’re just mismatched. Ask your supplier if they have a clear that they recommend that doesn’t craze. 

3)* Stroke and Coat isn’t an underglaze. It’s kind of it’s own weird animal. It behaves a lot like an underglaze in that it applies at green or bisque,  has a WYSIWYG appearance and doesn’t burn out or run, even at cone 10. (Some colours don’t survive the top end). But it’s technically sold as a glaze that has a really ridiculous, otherwise unheard of, super wide firing range. It’s supposed to have enough flux to get glossy without a clear coat over top at cone 06, but while it’ll be fused, it can be a little dry at that point. Lots of places do use a clear coat over top to help smooth out brushstrokes and cover any dry areas. But whenever you combine 2 glazes, you run the risk of them running more in tandem than they would separately. It’s a good idea to use clay cookies/wasters/drip catchers when combining glazes for the first time, even if you’re working with something as stable as S&C, and especially when you’re combining 2 different brands.

[LilBlueFrog]

Ok, thank you so much for the information. Now I have more questions!

What is WYSIWYG? I googled it but nothing obvious came up...

When you say the glaze and clay body are mismatched, what does that imply, chemically/physically? 

Is mixing brands an issue because brands might use different flux/stabilization ingredients that may then not play nicely together?

 

This was very helpful, thanks for the detailed reply!

[Bill Kielb]

3 hours ago, LilBlueFrog said:

I applied mayco stroke and coat to the leather hard piece,

Stroke and coat is a low fire glaze that matures at 04/05 and can be fired to cone 5 I believe, but ……. Mayco provides samples of expected color change at the higher temperature as well. So really the effect of combining two glazes is not always predictable or shall we say usually requires testing to figure out if the combination is workable.

Glazes craze when the rate of expansion / contraction of the glaze differs significantly from the clay. Testing is the only reasonable way to know, it cannot be calculated or predicted.

Edited September 14, 2022 by Bill Kielb

[LeeU]

WYSIWYG is "What you see is what you get".  Stroke & Coat does not always fire at mid-range the way the label says.  Also some labels do not mention mid-fire changes at all. I suggest testing each glaze a couple of times and make your own label for how it "really" does at 5-6...that will save disappointment & money & let you know which ones you can count on for expected results  on 5-6 bodies in a 5-6 fire. 

[Kelly in AK]

Also, just for good measure, it’s not cone 10 clay. The conchoidal shape of the chip on the bottom indicates your clay is well matured. 

[Min]

3 hours ago, Bill Kielb said:

Glazes craze when the rate of expansion / contraction of the glaze differs significantly from the clay. Testing is the only reasonable way to know, it cannot be calculated or predicted.

Well, hmmm,  let's just say our thoughts differ on whether crazing can be predicted.

 

[Bill Kielb]

57 minutes ago, Min said:

  let's just say our thoughts differ on whether crazing can be predicted.

Fair enough, just have never found a way or am aware of a way to calculate whether a glaze will craze or not, without testing.

[LilBlueFrog]

3 hours ago, Kelly in AK said:

Also, just for good measure, it’s not cone 10 clay. The conchoidal shape of the chip on the bottom indicates your clay is well matured. 

Ohhhh cool, thanks for the info! What shape does unmatured clay chip in? Also, are there ways to tell cone 5 vs cone 10 post-bisque? Asking for a friend, ahem. 

[Babs]

I am not seeing crazing on these photos.

Is it possible you applied glaze too thickly.

If the glaze moves, then so will underglaze it is fluxing with. Need a stiff clear imo

[LilBlueFrog]

12 hours ago, Babs said:

I am not seeing crazing on these photos.

Is it possible you applied glaze too thickly.

If the glaze moves, then so will underglaze it is fluxing with. Need a stiff clear imo

The external crazing is minimal but there, and yes its definitely possible that I put it on too thickly. 

I thought I was buying a stiff clear, but apparently not in this situation. Its good to know that the underglaze will follow the lead of the glaze in terms of movement. Thank you for the help!

[Callie Beller Diesel]

So there’s a lot to learn about glaze chemistry, and if you’re interested in furthering your knowledge, there are some good free resources. You are by no means required to have a big chemistry background if you’re making a few things just for fun, but knowledge helps you even if you’re just buying a few bottles of glaze and a box of clay and calling it therapy. If you want to get into selling functional work, it’s a good idea to learn a lot more though. 

You already know about this website, and there’s all kinds of fun and informative videos and articles to be found here. You have already discovered the forum

The reference we all know and love is digitalfire.com. It’s an older website, but it’s been put together by Tony Hansen, who makes all the clay and glazes for Plainsman, which is the main clay supplier in western Canada. It’s got articles and materials information galore. Here’s a link to the articles list there, but theres lots more to read. Apologies ahead of time to your family if you like going down internet rabbit holes. 

Glazy is also a great tool if you want to get into calculations, or to find glaze recipes to mix yourself, glazy.org offers a free glaze software calculator and a searchable database of glaze recipes others have posted. 

 

[Kelly in AK]

Breaks in less mature clay, or clay that’s really groggy, will be more jagged and rough.

I can’t think of a way to tell the difference at bisque stage. 

Edited September 14, 2022 by Kelly in AK

[LeeU]

On 9/13/2022 at 9:00 PM, Kelly in AK said:

conchoidal

Thanks Kelly-I either learned something new or got reminded of something I'd learned & forgotten. When I looked it up, the pronounciation sounded very familiar, tho the info itself not so much.

 

[LeeU]

On 9/14/2022 at 3:26 PM, Callie Beller Diesel said:

but knowledge helps

Amen sister--

[oldlady]

i am probably missing something important in the photos shown on my computer.  to me, the colors look as though someone added water to the underglaze.  the result is the very pale areas and the darker ones are where the water did not have much of an impact on the color.